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Refraction of light is a phenomenon that occurs when light travels from one transparent medium to another with a different density. This change in density alters the speed at which light travels, causing the light ray to change direction. The amount of bending or refraction that occurs is guided by Snell's Law. It is an everyday occurrence, noticeable when a straw appears bent in a glass of water or when a pool seems shallower than it actually is.

In our example, a ray of light encounters a dense medium, a glass block, from a less dense medium, air. Consequently, the path of the light ray bends towards the normal—the imaginary line perpendicular to the surface where light enters the glass. The degree of refraction not only depends on the angle at which light strikes the surface but also on the inherent properties of the materials involved, primarily their refractive indices, a concept we will explore shortly.

The angle of incidence is the angle between the incident ray of light and the normal to the surface at the point of contact. The normal is an essential reference point as it allows us to measure the degree to which light bends when transitioning between different mediums. In our textbook problem, the light ray strikes the glass at a 30-degree angle from the normal—a moderate angle that ensures the light will bend but not reflect off the surface.

Understanding the angle of incidence is crucial for predicting the behavior of light. This angle plays a pivotal role when applying Snell's law, as it helps determine the angle of refraction. The angle of incidence remains constant when light passes through a substance with parallel surfaces—like our glass block—unless the medium itself changes.

The refractive index, typically symbolized as 'n', indicates how much a substance can bend light. It's a ratio that compares the speed of light in a vacuum to its speed in the given material. For example, the refractive index of air is approximately 1.00 because light travels through air almost as fast as it does in a vacuum. In contrast, the refractive index of glass in our problem is 1.50, signaling that light travels more slowly in glass compared to air.The refractive index is a dimensionless number that plays a central role in Snell's law, a fundamental equation in optics. By knowing the refractive indices of both the original and the new mediums, we can predict exactly how much light will bend when entering or exiting a material. This property allows us to design lenses, glasses, and a multitude of optical instruments that improve and manipulate our vision and perception of the world around us.